Hornbeck refers to the tree of knowledge because it was a tree in the middle of the garden of Eden from which god forbade Adam to eat but a serpent tempted Eve to eat the forbidden fruit from the tree which then Adam later ate and so then they were banished from the garden. Hornbeck is saying this because Hornbeck asked if she wanted a bite of his apple and he was making fun of it because Hillsboro believes in the religious part of life.